Revealing physical activity of GRB central engine with macronova/kilonova data

0.21 MB·

by  Zhao-Qiang Shen

Draftversion January20,2017
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
REVEALING PHYSICAL ACTIVITY OF GRB CENTRAL ENGINE WITH MACRONOVA/KILONOVADATA
Zhao-Qiang Shen1,2, Zhi-Ping Jin1, Yun-Feng Liang1,2, Xiang Li1, Yi-Zhong Fan1, and Da-Ming Wei1
1 KeyLaboratoryofdarkMatterandSpaceAstronomy,PurpleMountainObservatory,ChineseAcademyofScience, Nanjing,210008,
China.
2 UniversityofChineseAcademyofSciences, YuquanRoad19,Beijing,100049, China.
Draft version January 20, 2017
7
ABSTRACT
1
ThemodelingofLi-Paczyn´skimacronova/kilonovasignalsgivesreasonableestimateontheneutron-
0
2 richmaterialejectedduringtheneutronstarmergers. Usuallytheaccretiondiskismoremassivethan
the macronovaejecta, with which the efficiencies of converting the disk mass into prompt emission of
n
three merger-driven GRBs can hence be directly constrained. Supposing the macronovae/kilonovae
a
associated with GRB 050709, GRB 060614 and GRB 130603B arose from radioactive decay of the
J
r-process material, the upper limit on energy conversion efficiencies are found to be as low as ∼
8 10−6−10−4. Moreover,forallthreeevents,neutrinoannihilationislikelypowerfulenoughtoaccount
1
for the brief gamma-ray flashes. Neutrino annihilation can also explain the “extended” emission

lasting ∼ 100 s in GRB 050709, but does not work for the one in GRB 060614. These progresses
]
E demonstrate that the macronova can serve as a novel probe of the central engine activity.
H Subjectheadings: gamma-rayburst: general—radiationmechanisms: non-thermal—binaries: general
— stars: neutron
.
h
p
- 1. INTRODUCTION the double neutron star binaries observed in the Galaxy
o (Lattimer 2012), the mass of the nascent black hole
The researches on mergers of a neutron star with ei-
r (M ) and its spin parameter (a) can be reliably rea-
t ther a neutron star or a stellar-mass black hole have at- BH
s tractedwider andwiderattentionsince they arepromis- sonably evaluated, as demonstrated in both analytical
a approaches and numerical simulations (Lee et al. 2000;
ingsourcesofsignificantgravitationalwave(GW)radia-
[
Kiuchi et al. 2009). AdoptingthesepriorvaluesofM
tion that are expected to be detectable for the advanced BH
anda, withthe electromagneticdatathe massofthe ac-
1 detectors such as LIGO and Virgo in the near future
v Abbott et al. (2016). Currently, the “related” obser- cretion disk (Mdisk) launching the sGRB ejecta can be
2 vational data are mainly for short Gamma-ray Bursts estimated (Fan & Wei 2011). Not surprisingly,the esti-
5 (sGRBs; the bursts with a duration shorter than 2 sec), matedaccretiondiskmassesaresensitivelydependentof
2 the energy extraction processes since the magnetic and
a kind of brief γ−ray transient widely believed to origi-
5 neutrinomechanismshaveratherdifferentenergyextrac-
natefromcompactobjectmergers(Eichler et al. 1989),
0 tion efficiency (Fan et al. 2005). As a result, there are
as favored by a broad range of observations (Berger
. large divergency between the accretion disk masses in-
1 2014). With the sGRB data, our understanding of the
ferredindifferentenergyextractionmodels (Fan & Wei
0 rate and the launched ultra-relativistic outflow (includ-
2011;Giacomazzo et al. 2013;Liu et al. 2015)andsuf-
7 ing the subsequent radiation) of neutron star mergers
1 have been significantly advanced (Nakar 2007; Berger fer from further uncertainties involved in the modeling.
: 2014). The knowledge of the central engine, such as The other hope is that for some “nearby” (∼ 100 Mpc)
v binary-neutron-starmergers,withthegravitationalwave
the accretion disk mass, the spin and the mass of the
i
X newly formed black hole, and the energy extraction effi- data the masses ofthe binaries as well as the formed ac-
cretion disk might be inferable in 2020s (Kiuchi et al.
r ciency,however,is almostsolelyfromthe numericalsim-
a ulations (Ruffert & Janka 1998; Rosswog et al. 1999; 2010). A short summary of the current situation is the
lackofsolidinformationaboutthecentralenginedirectly
Kiuchi et al. 2009; Rezzolla et al. 2011) because the
inferred from the observational data of sGRBs.
detectable photons were emitted at radii far from the
The neutron-rich sub-relativistic matter ejected in
central engine and usually the initial information of the
the neutron star mergers are the ideal sites to syn-
GRBejectawaslost. OneexceptionisfortheGRBswith
thesize the r−process material (Lattimer & Schramm
adistinctthermalradiationcomponentthestandardfire-
1974; Eichler et al. 1989). The radioactive de-
ballaccelerationandradiationapply(Piran et al. 1993;
cay of these heavy elements gives rise to the near-
M´esza´roset al. 1993), for which the initial radius at
infrared/optical transients, i.e., the so-called Li-
which the fireball was launched or reformed can be rea-
Paczyn´ski macronova/kilonova (Li & Paczyn´ski 1998;
sonablyinferredandaconstraintonthemassofthecen-
Metzger et al. 2010; Kasen et al. 2013). Recently
tralblackholeisimposed(Pe’er et al. 2007;Fan & Wei
macronova/kilonova signals have been identified in a
2011). However, no distinct thermal radiation compo-
few events (Tanvir et al. 2013; Berger et al. 2013;
nent has been identified in prompt emission of sGRBs,
Yang et al. 2015;Jin et al. 2015,2016). Suchnewtran-
yet.
sients, in addition to labeling the birth sites of the very
Thanks to the relatively narrow mass distribution of
heavy elements, are promising electromagnetic counter-
[email protected](YZF)[email protected](DMW) parts of gravitational sources that are expected to be
2
detectable by advanced LIGO/Virgo in the near future. resent the mergers of equal- and unequal-mass neutron
In this work we show that the macronova observations starsassumingseveraldifferentequationsofstate(EOS),
also serves as a novel probe of the central engine activity while the squares are the black hole-neutron star merg-
of sGRBs. ers with various initial parameters). One exception is in
the case of tidal disruption of a star with a hyperbolic
2. THEMETHOD orbit, half the material is ejected and another half is
Throughout this work we assume that the macrono- bound (i.e., M =M ; Hotokezaka 2016 private com-
disk ej
vae/kilonovae associated with GRBs arose from the ra- munication). Therefore we have M ≥ M if a black
disk ej
dioactive decay of the r-process material launched by hole was promptly formed in the mergers. For the neu-
neutron star mergers, which is particularly motivated tronstar−stellar-massblackholemergersitisalwaysthe
by the fact that a NS−BH merger model can reason- case. If instead a hypermassive neutron star was formed
ably reproduce the multi-epoch/band light curve of the in the binary neutron star mergers, it will take a time
macronova associated with GRB 060614 (see Fig.1 of ∼ 100 ms or longer to collapse into a black hole, during
Jin et al. 2015). An independent/important support which most of the disk material likely has accreted onto
to the scenario of generating ∼ 0.1 M⊙ r-process ma- the central remnant and Mej can not be taken as a ro-
terial in a neutron star merger event is the heavy ele- bust lower limit of M . That is why we focus on the
disk
ment enrichment detection of the ancient dwarf galaxy NS−BH mergers favored in current macronova/kilonova
Reticulum II (Ji et al. 2016). Alternatively the cur- modeling. AccordingtoFig.1itisalsoreasonabletotake
rent GRB-associated macronova/kilonova signals could Mdisk ≤0.3 M⊙.
be for example the thermal-like radiation of the sub- The energy extraction efficiency (ǫ) of the sGRB cen-
relativisticejecta heatedbythe centralengine-generated tral engine is
X-rays (Kisaka et al. 2016). In such a model the sub- ǫ=E /M c2, (1)
γ,j disk
relativisticejectamasscanbemuchlessthanthatfound
in the r-process material modeling and the progenitor where Eγ,j = Eγ(1−cosθj) ≈ Eγθj2/2 is the geometry-
stars may be double NSs. Nevertheless, as long as the corrected prompt emission energy of the sGRB, θ ≪ 1
j
ejecta mass is found in a given macronova/kilonova ori- is the half-opening angle of the GRB ejecta and E is
γ
gin model, the approaches outlined in this work can be the isotropic-equivalent energy of the prompt emission.
straightforwardlyappliedto. Henceinthisworkwefocus With the good-quality prompt and afterglow emission
on our “fiducial” case. data of some sGRBs, E can be directly measured and
γ
Within the r-process material radioactive decay sce- theθ canbeyieldedinthenumericalmodelingoftheaf-
j
nario,themodelingofthemacronova/kilonovalightcurve terglowemission,withwhichE is obtained. The main
γ,j
can yield a reasonable estimate of the mass of the challenge is how to measure or estimate M reliably.
disk
neutron-rich ejecta (M ) and possibly also distinguish FortunatelywehaveshownthatM ≥M ,withwhich
ej disk ej
betweentheprogenitorstars,eitherbinaryneutronstars ǫ can be constrained.
or neutron star-blackhole binary. The majordifferences
of the ejecta from these two kinds of progenitors in-
clude (Hotokezaka et al. 2013; Tanaka et al. 2014): a)
100
some NS−BH mergers eject much more material than Mej=Mdisk DB15
the NS binary mergers; b) NS-BH merger ejecta are 10Mej=Mdisk JB15 KK15
launched predominantly along the orbital plane. Conse- 10-1 Mdisk=0.3M KI15 FD14
⊙
quently, the macronovapoweredby some NS−BHmerg- NSNS
ersare muchmore luminous, bluer andlasts longerthan BHNS
the macronovae powered by NS binary mergers. Such 10-2
a goal likely has already been achieved in the studies )⊙
M
of some GRB/macronova events and NS−BH merger (
origin is favored (Hotokezaka et al. 2013; Yang et al. Mej10-3
2015; Jin et al. 2016; Kawaguchiet al. 2016). Below
we focus onprobing the centralengine activitywith M
ej
within the NS−BH merger scenario. 10-4
The central engine activity is “governed” by M .
disk
Previously the M can only be inferred with the
disk
electromagnetic data within the double neutron star 10-5
10-4 10-3 10-2 10-1 100
merger scenario (Fan & Wei 2011; Giacomazzo et al.
Mdisk(M )
2013; Liu et al. 2015), which is strongly-model de- ⊙
Fig.1.— The remnant disk rest masses and the ejecta masses
pendent. Such a puzzle, fortunately, can be solved
after binary compact object mergers in two latest numerical rel-
in a new way. As found in the advanced numer- ativity simulations. Triangles and squares represent the simula-
ical simulations on neutron star mergers (including tions of binaryneutron star (NSNS) mergers reported in Dietrich
both the double neutron star mergers and the neutron et al. (Dietrichetal. 2015) and the simulations of black hole-
neutronstar(BHNS)mergersreportedin(Kawaguchi etal. 2015;
star−stellar-massblackholemergers;see(Foucart et al.
Justetal. 2015; Kyutokuetal. 2015; Foucartetal. 2014), re-
2014; Dietrich et al. 2015; Kawaguchiet al. 2015; spectively. The dashed line is for Mdisk = Mej. A clear/general
Just et al. 2015; Kyutoku et al. 2015) and the refer- relationshipisMdisk≥Mej andthereislikelyalsoanupper limit
ences therein), there is a general conclusion that “the ofMdisk≤0.3M⊙.
accretion disk mass M is (significantly) larger than
disk
M ” (see Fig.1 for illustration; where the triangles rep- Thanks to the great efforts of the GRB follow-
ej
3
up observation community, three macronova signals In the Blandford−Znajek mechanism
have been identified in sGRB 130603B (Tanvir et al. (Blandford & Znajek 1977; Lee et al. 2000), the
2013; Berger et al. 2013), hGRB 060614 (hybrid GRB; luminosity of the electromagnetic outflow can be
(Yang et al. 2015; Jin et al. 2015)) and sGRB 050709 estimated by
(Jin et al. 2016). The numerical modeling of the signal
insGRB 130603Binthe binaryneutronstarmergersce- L ≈1.5×1052 erg s−1 ε ( a )2( M˙ )( θj )−2
nario suggests a Mej ∼ 0.03−0.08 M⊙ (Berger et al. BZ B 0.7 0.01 M⊙ s−1 0.1
2013), which might be a challenge since usually the bi-
[(1+p1−a2)/2]−2, (4)
naryneutronstarmergersarenotexpectedtobe ableto
eject such massive outflow. On the other hand, a neu- where ε ∼ O(1) is a dimensionless parameter to de-
B
tron star-black hole merger model (Tanaka et al. 2014) scribe the ratio between the ordered magnetic field en-
canreasonablyreproducethe macronovasignalinsGRB ergydensityandthetotalenergydensityoftheaccreting
130603B with a Mej ∼ 0.05 M⊙ (Hotokezaka et al. material. The energy extraction efficiency is thus
2013; Kawaguchiet al. 2016; Jin et al. 2016). For
hGRB 060614, the macronova modeling gives a Mej ∼ ǫ ≡ θj2 LBZ ≈3.8×10−4 εB( a )2[(1+p1−a2)/2]−2.
0.1 M⊙ in the neutron star-black hole merger scenario BZ 2 M˙ c2 0.1 0.7
and ∼0.2 M⊙ in the case of binary neutron star merger (5)
(Yang et al. 2015). The I/F814W-band macronova In the case of M˙ ≪ 1 M⊙ s−1 we have ǫνν¯ ≪ ǫBZ (see
signal of sGRB 050709 is rather similar to (though a
eq.(3) and eq.(5)).
bit dimmer than) that of hGRB 060614 and the neu-
tron star-black hole merger modeling suggests a M ∼ 3. CASESTUDIES
ej
0.05 M⊙ (Jin et al. 2016). Other physical parameters Below let us discuss these three GRBs and their “ex-
ofthesethreemacronova-associatedGRBs,includingE
γ tended emission” case by case. In particular we focus
and θ , are also summarized in Tab.1. With eq.(1) and
j on whether the neutrino process can work or not since
the fact that M ≥ M , it is thus straightforward to
disk ej all the ǫ reported in Tab.1 are sufficient small to be well
get the constraint on ǫ, which is found to be very low.
matched(see eq.(5)). To be able to accountfor the data
Let us check either the neutrino/anti-neutrino anni-
within the neutrino process the requests of L ≥ L
νν¯ τ
hilation process or the magnetic process is needed to (equally,ǫ ≥ǫ )andM˙ ≤M /τ shouldbesatisfied,
account for the data. Note that in typical neutron νν¯ τ disk
where(τ, L , ǫ )arethe(durationaftertheredshiftcor-
star-black hole mergers, an initial spin a > 0.6 and a τ τ
rection, luminosity, energy extraction efficiency) of the
MBH < 10M⊙ (the observed Galactic black holes likely prompt emission or alternatively the extended emission,
mhaavsesivaetnyepuictarolnm-raicshs o∼ut6flo−w8. TMh⊙er)efaorreenineetdheedfotlolowejiencgt respectively. Note that Mej ≤ Mdisk ≤ 0.3M⊙ and ǫτ
is different from ǫ in the case of presence of extended
discussion we take a=0.7 and MBH =7M⊙. An empir- emission.
ical relation of the luminosity of such a hot ejecta reads
GRB 050709. It is a short burst lasting ∼ 0.07 s
(Zalamea & Beloborodov 2011)
followed by the extended X-ray emission with a dura-
tion of ∼ 130 s. At a redshift of z = 0.16, the ini-
θ x M˙
Lνν¯≈5×1052 erg s−1 (0.j1)−2(1m.7s)−4.8(1 M⊙ s−1)9/4 trieaslpohnadrdingspliukmeihnaossitayEofγL∼ 2.8 ×∼10449.5e×rg10a5n0deragcso−r1-
τ=0.06 s
M (Villasenor et al. 2005). With eq.(2) we find out that
( BH)−3/2, (2)
7M⊙ withtheotherfiducialparameters,forM˙ ∼0.15M⊙ s−1
the neutrino/anti-neutrino process is energetic enough
which holds for M˙ < M˙ < M˙ , where x = to generate the hard spike. The corresponding ac-
ign trap ms
{3 + Z ∓ [(3 − Z )(3 + Z + 2Z )]1/2}/2 (where cretion disk mass is ∼ 0.01 M⊙, significantly smaller
2 1 1 2
Z = 1 + (1 − a2)1/3[(1 + a)1/3 + (1 − a)1/3] and than Mej ∼ 0.05 M⊙. Therefore most of the accre-
1 tion disk mass might have been consumed to yield the
Z = (3a2 + Z2)1/2; For a = (0.8, 0.9, 0.95) we
2 1 ∼ 112 (1+z) sec long-tail X-ray emission. The time-
have xms = (1.45, 1.16, 1.00), respectively); M˙ is averaged luminosity of the extended X-ray emission is
the accretion rate, M˙ = K (α/0.1)5/3, M˙ = L ∼ 3×1047 erg s−1, the neutrino process may
ign ign trap τ=112 s
K (α/0.1)1/3 and α is the viscosity. Both K and be able to workwith a highspinparametera∼0.9(i.e.,
trip ign
Ktrap are functions of a and for a = (0, 0.95), Kign = xms =1.16) and M˙ ∼2×10−3 M⊙ s−1. The disk mass
(0.071, 0.021) M⊙ s−1 and Ktrap = (9.3, 1.8) M⊙ s−1, for the extended emission is ∼ 0.22 M⊙, which may be
respectively (Zalamea & Beloborodov 2011). The cor- possible in some neutron star−black hole merger mod-
responding efficiency of “energy extraction” is els (see Fig.1). The ∼ 100 s duration of the “steady
extended” accretion process for the disk formed in the
θ2 L compact object mergers requires a α ∼ 10−3 (see also
j νν¯
ǫ ≡
νν¯ 2 M˙ c2 (Lee et al. 2009)), which is at the low end of the distri-
bution discussed in the literature (note that this request
≈7×10−6 (xms)−4.8( M˙ )5/4(MBH)−3/(23.) also holds for the magnetic energy extraction process).
1.7 0.1 M⊙ s−1 7M⊙ As far as the energy budget is concerned, the neutrino
process seems to be plausible for GRB 050709.
ThesmallertheM˙ is,thelessefficienttheenergyextrac- GRB 130603B. The burst was at a redshift of z =
tion we have. 0.356 and the prompt emission lasted for ∼ 0.12(1+z)
4
TABLE 1
The estimate of the energyextraction efficiency
GRB050709a GRB060614b GRB130603Bc
Eγ (1051 erg) 0.069 2.1 2.1
Duration(sec)d 0.07(+130) 5(+97) 0.18
θj (rad) 0.10 0.08 0.085
Mej (M⊙)e ∼0.05 ∼0.10 ∼0.05
Mdisk (M⊙) &0.05 &0.10 &0.05
ǫf .3.9×10−6 .3.8×10−5 .8.4×10−5
Lνν¯ forinitialspike Yes Possible Possible
Lνν¯ forextended emission Possible No
Note: a. Villasenoretal. (2005)andJinetal. (2016);
b. Gehrelsetal. (2006)andXuetal. (2009);
c. Tanviretal. (2013),Bergeretal. (2013)andFanetal. (2013);
d. Thedurationsincludethatofthehardspikeandthe‘extended emission’(inthebracket);
e. ThevalueofMej isestimatedwithintheneutronstar-blackholemergermodel;
f. Thevalueofǫisderivedwitheq.(1).
s with a luminosity of L ∼ 1.8 × 1052 erg s−1 a ∼ 0.7 the accretion disk masses M are required to
τ=0.12 s disk
(Tanvir et al. 2013). With eq.(2) we find out that with be ∼(0.005, 0.04, 0.12)M⊙ for the hardspikes of GRB
other adopted fiducial parameters, for M˙ ∼0.6 M⊙ s−1 050709, GRB 060614 and GRB 130603B respectively,
theneutrino/anti-neutrinoprocessisenergeticenoughto otherwise the neutrino process is not efficient enough.
generate the brief but intense gamma-ray flash (see also Though in some binary neutron star mergers Mdisk can
eq.(3)). This requires a Mdisk ∼ 0.07 M⊙, which seems be as massive as ∼ 0.2 M⊙ (see the triangles in Fig.1),
possible with an inferred Mej ∼0.05 M⊙ (see Fig.1). the inferred rather-low Mej hampers us to draw a reli-
GRB 060614. The prompt emission of hGRB 060614 able conclusion on the role of the neutrino process. For
at z =0.125 consisted of two epoches, the first is a hard the extended X-ray emission of GRB 050709 and GRB
spike lasting ∼ 4.4(1+z) sec with a Eγ ∼ 3.7×1050 060614, the required Mdisk ∼ (0.38, 1.6) M⊙ even for
erg (Xu et al. 2009), suggesting a corresponding lumi- a somewhat “optimistic a = 0.8. We thus suggest that
nosity L ∼ 8×1049 erg s−1. The neutrino/anti- the neutrino process is hard to account for the extended
τ=4.4 s
emission.
neutrino process can marginally give rise to the initial
gamma-rayflashwitha Mdisk ∼0.2M⊙ anda∼0.8. In
view of the rather massive ejecta with Mej ∼ 0.1M⊙, a
Mdisk ∼0.2M⊙ispossible. However,forthe∼86(1+z) 1053
sec long-tailemission of hGRB 060614with an averaged
luminosity of L ∼ 2×1049 erg s−1, the neutrino
τ=86 s 1052 GRB 130603B
process is hard to contribute. This is because the total
accretion disk is not expected to be more massive than
0.3 M⊙ (see Fig.1), with which the averaged accretion 1051
rate M˙ <3×10−3 M⊙ s−1. In the most promising case 1)− GhRaBrd 0 s5p0ik7e09
(tio.er.,enadveerryM˙higignh≤a∼100−.395Ma⊙nds−a1s)uffiweciehnatvsemaallluαmi≤no0s.i0ty1 L(ergs ¯νν1050 GhRaBrd 0 s6p0ik6e14 excluded region
of Lνν¯ ∼ 1048 erg s−1, one order of magnitude lower 1049 GRexBt e0n6d0e6d14
than the observed value. On the other hand, we have
wLBhZich≈c1a.n5×ma1t0c5h1 ethrgeso−b1seεrBv(aat/io0n.7d)2a(tMa˙ a/s10l−on3gMa⊙s εs−1>), 1048 τ=0.1 s a=0.7
B τ=1 s a=0.9
0.01(a/0.7)−2(M˙ /10−3M⊙s−1)−1. Theinitialmagneti- ττ==11000 s s GRB 050709
zationdegree(i.e.,theinitialratiobetweenthemagnetic 104710-2 10-1 extended 100
field energy density and the thermal energy density) of M˙τ (M )
the outflow powering the long-tail emission is expected ⊙
Fig.2.— The neutrino anti-neutrino annihilation luminosity
to be ∼ tens. Again, the ∼ 100 s duration of the accre-
(Lνν¯ forθj =0.1)as afunction ofM˙τ. The(solid, dashed, dash-
tion process for the disk formed in the compact object
dotted, dotted) lines are for τ =(0.1, 1.0, 10.0, 100.0) s, respec-
mergers requires a α∼10−3. tively. Thelinesandsymbolsin(black,red)arefora=(0.7, 0.9),
All these results/constraints in the case of neutrino respectively.
anti-neutrino annihilation are summarized in Fig.2.
Clearly the neutrino process may be able to account for
some but not all data and the magnetic process is in 4. CONCLUSIONANDDISCUSSION
principle sufficient for all the data.
We show in this work the macronova data provide a
IfMej ∼10−3−10−2M⊙,assuggestedinKisaka et al. novelprobeoftheGRBcentralengineactivity. Thecon-
(2016), the limits on the efficiencies of the central en-
versionefficiency of the disk mass into the GRB prompt
gines will be 1-2 orders of magnitude higher, leading
emission is found to be rather low and the neutrino
to looser constraints. For MBH ∼ 3M⊙ (i.e., within the process are likely able to generate the brief gamma-ray
double neutron star merger scenario), with a reasonable
flashes in all three events (see Tab.1 and Fig.2). The
5
situation is less clear for the extended emission found in Though the GRB/GW events are expected to be still
GRB 050709and GRB 060614. For the former,the neu- rare in the next decade, the macronova/GW events are
trino process may work thanks to the low luminosity of expectedtobemuchmorefrequent. Thestatisticalstudy
the extended X-ray emission. While for the latter, the can be helpful in revealing the macronova nature and
magnetic process is necessary. Note that these conclu- hence yield a better understanding of the GRB central
sionsarebasedontheNS−BHmergermodelforcurrent engine.
GRB-associated macronova/kilonova events. Interest-
ingly,allcurrentthreemacronova-associatedGRBshave ACKNOWLEDGMENTS
small offsets from the host Galaxy centers, in agreement We thank the referee for the helpful comments and
with the NS−BH mergermodelprediction(Troja et al. YZF appreciates Dr. K. Hotokezaka for the commu-
2008). In other scenarios that yield a much lower Mej, nications. This work was supported in part by 973
the constraints are weaker (see Sec.3). Nevertheless the Programme of China under grants No. 2013CB83700
NS−BHmergerscenarioiswelltestableintheeraofGW and No. 2014CB845800, National Natural Science
astronomy(Mandel et al. 2015). GRB060614,ifindeed Foundation of China under grants 11273063, 11433009
froma NS−BH merger,is within the expected advanced and 11525313, the Chinese Academy of Sciences
LIGO/VIRGO sensitivity range (Li et al. 2016a) and a via the External Cooperation Program of BIC (No.
further examination reveals a very promising detection 114332KYSB20160007).
prospect of NS−BH merger events (Li et al. 2016b).
REFERENCES
Abbott,B.P.etal.2016, LivingRev.Relat.19,1 Lee,W.H.,Ramirez-Ruiz,E.,&L´oopez-Ca´amara, D.2009,
Berger,E.2014, Ann.Rev.Astron.Astrophys.52,43 ApJL,699,L93
Berger,E.,Fong,W.,andChornock,R.2013, ApJL, 744,L23 Lee,H.K.,Wijers,R.A.M.J.,andBrown,G.E.2000, Phys.
Blandford,R.D.andZnajek,R.L.1977,MNRAS, 179,433 Rep., 325,83
Dietrich,T.,Bernuzzi,S.,etal.2015, Phys.Rev.D, 91,124041 Li,L.X.,andPaczyn´ski, B.1998,ApJL, 507,L59
Eichler,D.,Livio,M.,Piran,T.,&Schramm,D.N.1989, Li,X.,Li,X.,Hu,Y.M.,Fan,Y.Z.&Wei,D.M.2016,ApJ,
Nature, 340,126 827,75
Fan,Y.Z.andWei,D.M.2011, ApJ, 739,47 Li,X.,etal.2016, arXiv:1611.01760
Fan,Y.Z.etal.2013,ApJL, 779,L25 Liu,T.,Lin,Y.Q.,Hou,S.J.,andGu,W.M.2015,ApJ, 806,58
Fan,Y.Z.,Zhang, B.andProga,D.2005, ApJL, 635,L129 Mandel,I.,Haster,C.-J.,Dominik,M.,andBelczynski,K.2015,
Foucart,F.,Deaton, M.B.,etal.2014, Phys.Rev.D, 90, MNRAS,450,85
024026 M´esz´aros,P.,Laguna,P.,andRees,M.J.1993,ApJL, 415,181
Gehrels,N.etal.,2006, Nature, 444,1044 Metzger,B.D.etal.2010,MNRAS, 406,2650
Giacomazzo, B.,Perna,R.,Rezzolla,L.,Troja,E.,andLazzati, Pe’er,A.,Ryde,F.,Wijers,R.A.M.J.,M´esz´aros,P.,&Rees,M.
D.2013, ApJL, 762,L18 J.2007,ApJL, 664,L1
Hotokezaka, K.,etal.2013,ApJL, 778,L16 Piran,T.,Shemi,A.,&Narayan,R.1993,MNRAS, 263,861.
Ji,A.P.,Frebel,A.,Chiti,A.,&Simon,J.D.2016,Nature, 531, Nakar,E.2007, Phys.Rep.442,166.
610 Rezzolla,L.,Giacomazzo, B.,Baiotti,L.,Granot,J.,
Jin,Z.P.etal.2015,ApJL, 811,L22 Kouveliotou, C.,&Aloy,M.A.2011,ApJL, 732,L6
Jin,Z.P.etal.2016, Nat.Commun., 7,12898 Rosswog,S.,Liebend¨orfer,M.,Thielemann,F.-K.,Davies,M.B.,
Just,O.,Bauswein,A.,etal.2015,MNRAS, 448,541 Benz,W.,&Piran,T.1999,A&A, 341,499
Kasen,D.,Badnell,N.R.&Barnes,J.2013, ApJ, 774,25 Ruffert,M.,&Janka, H.-Th.1989,A&A, 338,535
Kawaguchi,K.,Kyutoku,K.,etal.2015, Phys.Rev.D, 92, Tanaka,M.etal.2014, ApJ, 780,31
024014 Tanvir,N.R.etal.2013, Nature, 500,547-549
Kawaguchi,K.,Kyutoku,K.,Shibata,M.,andTanaka,M.2016, Troja,E.,King,A.R.,OBrien,P.T.,Lyons,N.,&Cusumano,
ApJ, 825,52 G.2008,MNRAS,385,L10
Kisaka,S.,Ioka,K.,andNakar,E.2016,ApJ, 818,104 Villasenor,J.S.etal.2005, Nature, 437,855
Kiuchi,K.,Sekiguchi, Y.,Shibata, M.,andTaniguchi,K.2009, Xu,D.etal.2009,ApJ, 696,971
Phys.Rev.D, 80,064037 Yang,B.etal.2015, Nat.Commun., 6,7323
Kiuchi,K.,Sekiguchi, Y.,Shibata, M.,andTaniguchi,K.2010, Zalamea,I.,&Beloborodov,A.M.2011,MNRAS,410,2302
Phys.Rev.Lett., 104,141101
Kyutoku,K.,Ioka,K.,etal.2015, Phys.Rev.D, 92,044028
Lattimer,J.M.2012, Annu.Rev.Nucl.Part.Sci., 62,485
Lattimer,J.M.andSchramm,D.N.1974, Astrophys.J.Lett.
192,L145